Two children at once –

The origin of life is one of the wonders of nature. For the new person to develop, he needs the mother’s egg and the father’s sperm. The egg cells in the mother’s ovary mature until they reach the fallopian tubes through ovulation, where they can be fertilized by a male sperm. This results in a so-called zygote, a cell that now contains the father’s genome as well as the mother’s DNA in the cell’s nucleus.

The fertilized egg travels through the fallopian tube to the uterus to implant and grow. During this migration, the zygote begins to divide, so that more and more connected cells are formed, and a complex of two, then four, eight, sixteen cells and more grows. After about seven days, the cells begin to specialize into different types of cells. At this point, the so-called blastocystThe group of cells travels from the fallopian tube to the uterus and nests there to continue growing.

cellular play of colors

Fraternal twins consist of two different fertilized eggs, while identical twins consist of only one fertilized egg. National Human Genome Research Institute / Public Domain

… or even two

By chance, it may sometimes happen that this group of cells does not “clinch” properly on its way to the uterus and divides into two different groups of cells. This can happen right in the beginning, after the first division of the zygote, and up to the blastocyst stage. In very rare cases even later. After this division, the two new cell groups continue to migrate and grow separately from each other. In this way, organisms develop from a fertilized egg cell, which thus possesses identical genetic information. The result is identical monozygotic twins.

On the other hand, odd or dizygotic twins occur when two different egg cells mature at the same time, are fertilized in the fallopian tubes by different sperms and then migrate independently to the uterus. Because egg and sperm cells are different, these twins do not have identical genetic material, but are closely related like normal siblings.

Siamese and semi-identical twins

In very rare cases, when monozygotic twins are formed, the cell structure can divide “too late”, about twelve days after fertilization, and therefore cannot be completely separated. As a result, these twins continue to grow together and are born “grow together”. In most cases, Siamese twins are connected at the sternum, but there are also many other options such as connecting via the head, hips, or coccyx.

As long as they don’t share any vital organs, conjoined twins can be separated through surgery. However, sometimes they share one heart, two lungs, or one brain, making separation impossible. Siamese twins have been around since humans were around. But they only got their name from a pair of twins born in Siam in 1811, who became known around the world as a circus attraction.

Even a rare and absolute condition among identical twins is near-identical twins. Normally, due to the protective mechanism, the egg cells are designed to be fertilized by only one sperm. This is the only way to ensure that the chromosome set is correct and the zygote is viable. In the case of semi-identical twins, on the other hand, one egg cell is fertilized by two sperm cells.

The zygote can only survive if division occurs in such a way that the set of chromosomes is correct again. Therefore, this type of twin is extremely rare: to date, only two cases are known worldwide. These two pairs of twins have 100 percent the same genetic information from the mother, as they were formed from the same egg cell as normal identical twins, but they inherited different genetic information from the father due to the two sperms.

placenta proportions
Twins can share the amniotic sac and placenta in different ways. © Kevin Dufendach (2008) / CC-by-sa 3.0

“WG” in the womb

When twins grow up in the womb, they must share their protective and nurturing home. Various forms of “living community” can occur before birth. Normally, the baby grows in an amniotic sac that consists of two layers of skin and is supplied by the placenta. In contrast, Geminis can sometimes share these spaces and resources in different ways. However, this depends on their origin and evolution.

When each twin has a placenta and an amniotic sac, this condition is called diplopia, because the placenta originates from what is called the placenta and the amniotic sac is the inner layer of the skin of the amniotic sac. This is how all fraternal twins and a third of identical twins grow.

However, this differs for identical twins who later separate. By the time separation occurs, the chorion, and rarely the amnion, has already begun to form. Depending on when the separation occurred, these components no longer appear twice, but only once for both twins prior to separation. The most common are monochorionic twins with an amnion, which separate four to seven days after fertilization. These then share the placenta, but each develops its own amnion and thus its own amniotic sac. In one percent of cases, the twins separate so late that the chorion and amnion are already developed, so the twins have to share a placenta and amniotic sac. Mono-placental mono-amnion twins are formed.

The split proportions of the amniotic sac and placenta are checked in each twin pregnancy, as they can pose different risks. A shared placenta can lead to decreased supply and poor development of the twin. In the case of a mono-placental pregnancy, there can also be a risk of Siamese twins or umbilical cord coils. Last but not least, this scan can also provide information about whether the twin is monozygotic or dizygotic.

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